Science

Big Flightless Birds Come From High-Flying Ancestors

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Big, flightless birds like the ostrich, the emu and the rhea are scattered around the Southern Hemisphere because their ancestors once flew around the world, a new study suggests.

That's a surprise, because it means birds in Australia, Africa and South America independently evolved in ways that made them all lose the ability to fly.

These related birds — known as ratites — have been a puzzle for evolutionary biologists for more than a century.

"The mystery was, how did they turn up on all the southern continents, and why were they so large — these great, big flightless things?" says Alan Cooper, director of the Australian Centre for Ancient DNA at the University of Adelaide. "They don't do any of the conventional bird things. They don't fly, and they're fairly unspectacular-looking in many ways, apart from the fact that they're huge."

Scientists used to believe that these birds shared a flightless ancestor that once lived on a supercontinent called Gondwana that was in the Southern Hemisphere back in the Cretaceous — millions of years ago.

The idea was that, as this supercontinent broke up into pieces, "the ratite birds were sitting on board and were being separated by these continents moving, and that that's how they came to be where they were," says Cooper. "And because they're flightless, it seemed like a pretty good model."

Pretty good — but also wrong. Or at least that's what Cooper now thinks, because of a recent discovery.

He and some colleagues extracted DNA from the bones of an extinct bird that lived on the island of Madagascar until about 1,000 years ago. It's called the "elephant bird," because Arab traders saw the remains of its giant eggs and imagined the bird that must have laid them.

"The legend was that this bird would carry off elephants," says Cooper. "That's how the elephant bird actually got its name."

The elephant bird couldn't fly and looked like an ostrich — except about 30 percent bigger. When Cooper's team compared the ancient bird's DNA to all the other flightless birds in the ratite group, they found that its closest relative was the kiwi, the little national bird of New Zealand.

Cooper was shocked. He says the kiwi was "the last one I would have predicted if I had to guess." He says the tiny, insect-eating kiwi looks nothing like the giant, plant-eating elephant bird.

Importantly, he says, "it's completely on the other side of the world." That means there's no way that separating continents can explain why a bird in Madagascar has its closet relative in New Zealand.

"And you're looking at this result and thinking, 'It has to be flying!' You can't get from Madagascar to New Zealand any other way," says Cooper.

He believes small ancestral birds must have flown long distances, taking up residence in new places. These small birds then independently evolved into the big, flightless birds we see around the Southern Hemisphere today, according to his team's report in the journal Science.

"No one would ever have expected it," says Allan Baker, a researcher at the Royal Ontario Museum in Toronto. He used to believe that separating continents explained the evolution of these birds, but now he agrees that they must have had flying ancestors.

"You can't close your mind to this. And so when other evidence comes up that points this out, you have another hypothesis that you're testing and it appears to be a much better explanation of what's going on," says Baker. "I think that's fabulous. It's how science progresses."

One remaining question is why these birds around the world would independently lose the ability to fly and get big.

Cooper notes that this happened right after the extinction of the dinosaurs 65 million years ago. Between that event and the eventual rise of mammals, there would be a time with no predators, when birds could become big and flightless without being eaten.

And I'm Audie Cornish. You may have noticed that big, flightless birds are scattered across the continents of the southern hemisphere. Africa has the ostrich, Australia has the emu and South America has the rhea. Well, scientists used to think they knew how these crazy-looking birds got to where they are today. Now, as NPR's Nell Greenfieldboyce reports, they're astonished to learn just how wrong they were.

NELL GREENFIELDBOYCE, BYLINE: The ostrich is probably the most famous member of a group known as the ratites - primitive-looking, flightless birds. Evolutionary biologists have puzzled over them for more than a century.

ALAN COOPER: And the mystery was how did they turn up on all the southern continents, and why were they so large, these great big flightless things.

GREENFIELDBOYCE: Alan Cooper is director of the Australian Centre for Ancient DNA at the University of Adelaide. He says ratite birds, like the ostrich and the emu are just weird.

COOPER: They don't do any of the conventional bird things. They don't fly, and they're fairly unspectacular-looking in many ways, apart from the fact they're huge.

GREENFIELDBOYCE: Scientists used to think all of these large, flightless birds descended from a large, flightless ancestor that lived millions of years ago when the southern hemisphere had a giant supercontinent. That supercontinent broke up - Africa drifted away, then India and South America. Cooper says people thought that if these land masses moved...

COOPER: The ratite birds were sitting on board and were being separated by these continents moving and that that's how they came to be where they were. And because they're flightless, it seemed like a pretty good model.

GREENFIELDBOYCE: Pretty good, but also wrong - or at least that's what Cooper now thinks, because of a recent surprise discovery. He and some colleagues extracted DNA from the bones of an extinct bird called the elephant bird. It lived on the island of Madagascar until about 1,000 years ago. It looked like an ostrich, only about 30 percent bigger. It didn't fly, obviously. When Cooper's team compared its DNA to all the other birds in this flightless group, they found that its closest relative was the kiwi.

COOPER: The last one I would have predicted if I had to guess that it was going to be related to would be the kiwi.

GREENFIELDBOYCE: Cooper was shocked. The kiwi is a chicken-sized bird that lives in New Zealand, a place with no geographic connection to Madagascar.

COOPER: And you can't have a continent or route to get from Madagascar to New Zealand. It's just completely the opposite side of the world and there's never been a close relationship between these two places.

GREENFIELDBOYCE: So, how did a bird get from one place to the other? He was left with only one explanation.

COOPER: It has to be flying. You can't get from Madagascar to New Zealand any other way.

GREENFIELDBOYCE: He says small ancestral birds must have flown long distances, taking up residence in new places and then independently evolving into the big, flightless birds we see around the southern hemisphere today. All of this is reported in the journal Science.

ALLAN BAKER: No one would ever have expected it.

GREENFIELDBOYCE: Allan Baker is a researcher at the Royal Ontario Museum in Toronto. He used to believe that separating continents and not flying explained the history of these birds, but says this new evidence made him think again.

BAKER: You can't close your mind to this. And so when other evidence comes up that points this out, you know, you have another hypothesis that you're testing and it appears to be a much better explanation of what's going on. So, I think that's fabulous. It's how science progresses.

GREENFIELDBOYCE: One remaining question is why these birds around the world would all independently lose the ability to fly and get big. Cooper notes that this happened right after the extinction of the dinosaurs. Between that event and the eventual rise of mammals, there would be a time with no predators, when birds could let themselves become big and flightless, with no danger of being eaten. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.